Soggier forest soils thwart the uptake of climate-warming methane

A recent investigation has revealed that the ability of forest soils to absorb methane has declined over time, likely due to an increase in precipitation as a result of climate change.

The authors of a new study found that methane uptake declined by as much as 89 percent, and a review of the scientific literature demonstrated that the phenomenon was taking place around the world.

These findings suggest that current carbon budgets may be overestimating the amount of methane, a far more potent greenhouse gas, that forest soils can siphon from the atmosphere, the scientists write.

Rainier weather could be hampering the ability to siphon methane from the air by forest soils, thought to be important repositories for the growing concentrations of the greenhouse gas in the atmosphere, new research shows.

“Our data strongly suggests that we’re significantly overestimating how much methane is being captured by forest soil,” Peter Groffman, one of the study’s authors and a biogeochemist at the City University of New York, said in a statement. “[That] means a lot more of this greenhouse gas is ending up in our atmosphere a lot faster than we believe.”

Until now, scientists have generally seen the bacteria-rich soils of forests as a reliable sink for the methane we’ve been pumping into the air, with some hinting that global warming might even be increasing these soils’ capacity to take in methane. Groffman and Ni wanted to figure out if that was really the case.

“We were interested in how methane uptake by forest soils was influenced by environmental change,” Groffman said in a second statement. “Do things like soil temperature, nitrogen, or rainfall impact forest soil’s ability to act as a methane sink? And how does this play out over time?”

To answer those questions, the duo collected up to 18 years of data from a variety of long-term study forests in the United States, ranging from rural to urban to calcium-enriched forests. The researchers then measured the amount of methane take up by the soil at each site over time, as well as the moisture and nitrogen content and the activity of microbes in the soil.

Forest soil methane uptake has been recorded at Hubbard Brook Experimental Forest in the White Mountains of New Hampshire since 2002. Image by Claire Nemes.

The rises in temperature and methane in the air, in combination with lower nitrogen levels in the soil, indicated that the soils’ rate of methane intake should have increased. But instead, they found a drop-off of 53 to 89 percent in the amount of methane they were able to absorb.

The team was also curious as to whether this decline was something specific to just these forests, or if they might be able to find evidence for it elsewhere in the world. So they combed through more than 300 studies on the interplay between methane and forest soils going back to the 1980s up through 2015. During that time period, they found, the absorption of methane by soils had fallen by 77 percent in soils found in forests around the world.

A common factor between both the researchers’ field data and the information from the scientific literature was that precipitation had increased across the board, suggesting that the higher rainfall instigated by climate change might continue to stifle the removal of methane from the atmosphere.

“We can’t rely on natural processes to solve our greenhouse gas problems,” Groffman said. “Just as trees and oceans may not always be able to absorb carbon dioxide, forest soils may not always be able to take up methane and keep it out of the atmosphere.”

Baltimore field lead Dan Dillon collects a gas sample. Image by Laura Templeton.

This new information could help in planning for an uncertain future, Ni said in the statement.

“Long-term changes in precipitation and forest soil methane uptake should be factored into models being used to inform policy decisions around methane-producing activities — to ensure that we’re using the most accurate tools available to account for methane sources and sinks,” Ni said.

And beyond just accounting for them, we also need to do what we can to avoid releasing methane where possible, said Steve Hamburg, a scientist with the Environmental Defense Fund, who was not involved in the research.

“Towards that end, we need a better understanding of the global methane budget and the causes of the increases in atmospheric concentrations,” Hamburg said in the statement. “Understanding that the global forest soil sink is weakening is a potentially important piece of the puzzle.”

Banner image of Hubbard Brook Experimental Forest in New Hampshire by Claire Nemes.